Polyamic acid precursors and methods for preparing higher molecular weight polyamic acids and polyimidebenzoxazole

ABSTRACT

Polyimide precursor solutions comprise an organic liquid and the reaction product of an aromatic dianhydride and an aromatic diaminobenzoxazole capped, on at least one terminal end, with a bifunctional chain extender. The bifunctional chain extender has one functional group reactive with the amine of the aromatic diaminobenzoxazole or the anhydride of the aromatic dianhydride and another functional group which does not form amic acid linkages, but which is capable of further reaction to increase the molecular weight of the polyimide precursor under conditions other than those used to react the aromatic diamine and aromatic dianhydride to form the polyimide precursor. These polyimide precursors can be converted into polyimidebenzoxazole polymers.

BACKGROUND OF THE INVENTION

The present invention relates to polyimide precursors, to a method forconverting these precursors into higher molecular weight polyamic acidsand/or polyimidebenzoxazole ("PIBO") polymers, and to films and coatingsof PIBO.

In the preparation of polyimidebenzoxazole, a dianhydride anddiaminobenzoxazole are first reacted to form a polyamic acid. Thepolyamic acid is then converted to PIBO by closing the amic acidlinkages to form imide rings. In general, the polyamic acids areprepared as a solution in an organic solvent such asN,N'-dimethylacetamide or the like. Heretofore, only a limited number ofdianhydride/diaminobenzoxazole combinations have been disclosed as beinguseful for producing PIBO.

U.S. Pat. No. 4,087,409 to Preston broadly teaches reacting twosymmetrical monomers to produce an essentially linear heterocyclicpolymer having at least two different heterocyclic linkages. Polyamicacids can be prepared by selecting monomers from an otherwise long listof monomers (e.g. 2,2'-p-phenylene bis(5-aminobenzoxazole) andpyromellitic dianhydride). See also, the articles "Fibers from OrderedBenzheterocycle-Imide Copolymers," Appl. Poly. Sym., No. 9, pp. 145-158(1969) and "New High-Temperature Polymers, VIII. Ordered Benzoxazole-and Benzothiazole-Imide Copolymers," J. Poly. Sci., Part A-1 (1969),7(1), pp. 283-296.

"Azole Analogs of Polypyromellitimides," Vysokomol. soed., Vol. (A)XIII, No. 11, 1971, pp. 2565-2570, discloses synthesis of azolecontaining analogs of poly-N,N'-(p,p'-phenoxyphenylene)pyromellitimide!. A PIBO prepared from2,6-di(p,p'-aminophenoxyphenyl)benzo 1,2-d:5,4-d'!bisoxazole andpyromellitic dianhydride is described.

U.S. Pat. No. 4,866,873 to Mukai et al., discloses an aromaticheterocyclic polyimide comprising substantially equimolar amounts of aspecific aromatic, trans-benzobisoxazole or trans-benzobisthiazolediamine such as 2,6-(4,4'-diaminodiphenyl)benzo 1,2-d:4,5-d'!bisoxazoleand a specific aromatic tetracarboxylic dianhydride such as pyromelliticdianhydride. These polyamic acid precursors are prepared in an amidesolvent and converted to PIBO. Similar PIBO polymers prepared from apolyamic acid solution are taught in Japanese Pat. Appl. No. 2-41819 toMitsubishi Kasei Corporation and in "Novel Aromatic HeterocyclicPolyimide (PIBT) Having Ultra-High Modulus of Elasticity" by SeiichiNozawa, Kagaku to Kogyo, 44(7), 1154 (1991).

"Synthesis and Mechanical Properties of Novel Polyimide ContainingHeterocycles" by Nozawa, Taytama, Kimura and Mukai presented at the 22ndInternational SAMPE Technical Conference (held Nov. 6-8, 1990), teachessynthesis of one type of trans-PIBT or trans-PIBO using a polyamic acidprecursor of 4,4'-diaminophenylenebenzobisthiazole and pyromelliticdianhydride. Japanese Patent Application No. 41-42458 (Patent KOHO No.45-8435) to Taoka Senryo Seizo K. K. teaches the synthesis of PIBO froma polyamic acid with the polyamic acid being prepared from an aromaticdiamine containing a single benzoxazole ring such as5-amino-2-(4-aminophenyl)benzoxazole and an aromatic tetracarboxylicacid dianhydride using thermal ring-closure techniques.

While some of the described PIBO polymers have been formed into fibersand films, they do not generally have sufficient physical and/orchemical properties (such as tensile modulus, tensile strength,elongation-to-break and coefficient of thermal expansion (CTE)) to makethem generally useful in applications such as for the electronicsindustry.

In addition, polyamic acids used in preparing PIBO polymers can formhighly viscous solutions at relatively low solids content. For example,solutions of polyamic acid containing as little as 7 weight percent of ahigh molecular weight polyamic acid can exhibit a viscosity of 100,000centipoise (cps) or more. For various applications such as casting offilms using conventional techniques, these high viscosities can beacceptable; for other applications such as spin-casting of coatings,lower viscosities are required or desired.

European Patent Publication No. 0 355 927 to Asahi discloses aphotopolymerizable polyimide precursor comprising a specific polyamicacid ester, polyamic amide or polyamic acid salt structure derived froma tetracarboxylic acid compound, a specific diamine compound and analcohol such as 2-hydroxyethyl acrylate or an epoxy compound such asglycidyl methacrylate. Each of the polymer repeat units of the describedpolymer contains a pendant organic group having 1 to 20 carbon atoms.The reference also suggests that when a group such as --C.tbd.N or--C.tbd.CH or a maleic anhydride residue is introduced into theprecursor as a terminal group, the molecular weight of the ultimatepolyimide produced by heat-curing can be increased. The polymers form acomposition photopolymerizable with a photopolymerization initiator. Theresulting PIBO polymer can be used to coat substrates for subsequent usein photolithography. Films produced from these cross-linked polymersexhibit relatively low tensile strengths and tensile modulus.

In view of the deficiencies in the prior art, it remains desirable toprepare polyimide precursor solutions from which PIBO polymers having adesired combination of chemical and physical properties can be prepared,including, when desired, a polyimide precursor solution having a highersolids content at acceptable viscosities for film forming applications,e.g., spin-casting. In addition, it remains desirable to convert thesePIBO polymers into useful films and coatings.

SUMMARY OF THE INVENTION

Accordingly, in one aspect, the present invention is a polyimideprecursor comprising a reaction product of an aromatic dianhydride andan aromatic diaminobenzoxazole which reaction product is terminated, onat least one but not more than two chain end(s), with a bifunctionalchain extender having one functional group reactive with the amine ofthe aromatic diaminobenzoxazole or the anhydride of the aromaticdianhydride and having another functional group which does not form amicacid linkages and, which is capable of further reaction to increase themolecular weight of the reaction product or the PIBO prepared therefromunder conditions other than those used to react the aromatic diamine andaromatic dianhydride to form the polyimide precursor.

In a preferred embodiment, the aromatic dianhydride is of the formula:##STR1## and the aromatic diaminobenzoxazole is of the formula: ##STR2##where Ar, Ar¹, Ar², Ar³, and Ar⁴ are an aromatic group or pyridine groupand the bifunctional chain extender is H₂ N--M--C(R)═C(R)₂, H₂N--M--C.tbd.C(R), ##STR3## where M is a divalent organic radical,preferably alkylene or arylene; and R is hydrogen, alkyl or aryl. Thepolyimide precursor is typically prepared as a solution in an organicliquid.

A PIBO polymer can be prepared from the polyimide precursor by exposingthe precursor at conditions sufficient, either simultaneously orsequentially, to cyclocondense (imidize) the precursor and increase itsmolecular weight and in another aspect, the present invention is amethod for preparing a PIBO polymer by heating the polyimide precursor.While imidization and an increase in molecular weight or chain extensionmay occur at substantially the same or widely different temperatures, ingeneral, the polyimide precursor is subjected to a conversiontemperature(s) from about 160° C. to about 280° C. to convert at least aportion of the precursor to polyimidebenzoxazole or a higher molecularweight polyamic acid (PA).

In a preferred embodiment, the PA/PIBO is further treated by exposing itto an additional temperature higher than the conversion temperature andfrom about 250° C. to about 600° C. for from about 0.1 to about 300minutes. This further heat-treatment can comprise exposing the PA/PIBOto a single, or two or more different, successively higher temperatures.In a most preferred embodiment, following exposure to the conversiontemperature, the PA/PIBO is exposed to (i) a single temperature ofgreater than about 350° C. and less than about 600° C. for about 0.1 toabout 120 minutes or, alternatively, to (ii) an annealing temperaturefrom about 250° C. to about 400° C. for from about 0.1 to about 120minutes and then a heat-treating temperature, higher than the annealingtemperature, from about 260° C. to about 600° C. for from about 0.1 toabout 120 minutes.

In yet another aspect, the present invention is a method for preparing apolyimidebenzoxazole film which comprises the steps of:

a) forming a liquid film of a polyimide precursor solution;

b) removing at least a portion of the organic liquid employed as thereaction medium from the polyimide precursor solution; and

c) heating the polyimide precursor at a conversion temperature fromabout 160° C. to about 280° C. for a period of from about 5 to about 90minutes.

Optionally, the method comprises an additional step (d) in which thepolyimide precursor is further exposed to a temperature higher than theconversion temperature and from about 250° C. to about 600° C. for about0.1 to about 300 minutes. In a preferred embodiment, the liquid film iscontacted with a ring-closure agent prior to or while at a temperatureat which imidization can occur.

In still another aspect, the present invention is a polyimidebenzoxazolefilm made using the described process.

The polyimide precursors can be prepared in a controlled manner by thereaction of the aromatic dianhydride, the aromatic diaminobenzoxazole,and the bifunctional chain terminator without an alcohol or epoxyreactant and in the absence or essentially absence of same. Themolecular weight of the reaction product and its solution viscosity canbe controlled by the time and temperature of the reaction as well as thestoichiometry of the reactants. The resulting polyimide precursorproduct is soluble in many common organic solvents such asN-methylpyrrolidinone and N,N'-dimethylacetamide.

A PIBO polymer is prepared by heating the polyimide precursor. As such,the polyimide precursor can be directly converted from a soluble,processable material to the finished, PIBO polymer. Hence, the method ofthe present invention facilitates the preparation of finished PIBOpolymer articles. For example, in the preparation of coatings, thesoluble precursor can be applied to a substrate or article being coatedand subsequently and directly converted into the desired PIBO product.

The PIBO polymers prepared from the precursors exhibit an excellentbalance of chemical and physical properties, including a goodcombination of strength, modulus and elongation-to-break and goodresistance to environmental influences. By selecting the type and amountof monomers and specific conditions of preparation, it is possible tomodify and design these properties. The PIBO products can be prepared asrigid and stiff or more flexible materials, depending on the specificmonomers selected and desired end use. The synthesis techniques formaking polyimide precursors and processing techniques for converting thepolyamic acid into PIBO results in polymers having good molecularweights and films and coatings with excellent properties, includingexcellent tensile properties; electrical properties such as dielectricconstant, dissipation factor, break down voltage and arc tracking, anddimensional stability properties.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, polyimide precursors are prepared by reactingan aromatic dianhydride, an aromatic diaminobenzoxazole and abifunctional chain terminator. The aromatic dianhydride and an aromaticdiaminobenzoxazole suitably employed in the practice of the presentinvention are those dianhydrides and diamines which are capable ofreacting to form a polyamic acid.

Representative aromatic dianhydride include compounds of the followingstructural formula: ##STR4## in which Ar can be an aromatic, includingpolyaromatic and fused aromatic, or inertly substituted aromatic,wherein "inertly substituted aromatic" means an aromatic having one ormore substituent groups such as a halogen which is essentially inert,preferably inert, to reaction with the aromatic diaminobenzoxazole. Forexample, Ar can be: ##STR5## where T is --O--, alkylene, --S--, --CF₂--, --SO₂ --, --CH₂ --, ##STR6##

The most preferred dianhydrides are pyromellitic dianhydride ("PMDA"):##STR7## 4,4'-oxydiphthalic anhydride ("ODPA"), ##STR8##2,2-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride ("6FDA"),##STR9## 3,3',4,4'-biphenyl tetracarboxylic dianhydride ("BPDA"),##STR10## 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride (DSDA),and ##STR11## 3,3',4,4'-benzophenonetetracarboxylic dianhydride##STR12##

Representative diaminobenzoxazoles useful in the practice of the presentinvention include compounds of the following structural formula:##STR13## where Ar¹, Ar², Ar³, and Ar⁴ are an aromatic group, inertlysubstituted aromatic group, or pyridine group.

Preferred diaminobenzoxazole monomers include: ##STR14## when Ar¹ is:##STR15## where T¹ is --O--, --S--, --SO₂ --, --S(═O)--, --CH₂ --,--Si(R)₂ --, ##STR16## and each Ar² are the same or different and are:##STR17## and T² is --O--, --SO₂ --, --S--, --S(═O)--, --CH₂ --,--Si(R)₂ --, ##STR18## when Ar³ is: ##STR19## and Ar² is as hereinbeforedescribed; and ##STR20## when each Ar³ are the same or different and Ar⁴is: ##STR21## and T⁴ is: --S--, --O--, SO₂ --, --CH₂ --, --HC═CH--##STR22##

The most preferred diaminobenzoxazole monomers are2,6-(4,4'-diaminodiphenyl)benzo 1,2-d:5,4-d'!bisoxazole ("DABO"):##STR23## 5-amino-2-(p-aminophenyl)benzoxazole ("p-DAMBO"), ##STR24##5-amino-2-(m-aminophenyl)benzoxazole ("m-DAMBO"), ##STR25##4,4'-diphenylether-2,2'-bis(5-aminobenzoxazole) ("OBA(DAMBO)₂ "),##STR26## 2,2'-p-phenylenebis(5-aminobenzoxazole) ("TA(DAMBO)₂ "),##STR27## and2,2-bis(4-phenyl)hexafluoropropane-2,2'-bis(5-aminobenzoxazole)("6FA(DAMBO)₂ "). ##STR28##

The bifunctional chain extender is a compound with one functional groupreactive with either the amine of the aromatic diaminobenzoxazole or theanhydride of the aromatic dianhydride and a second functional group(hereinafter referred to as a "latent moiety") which does not form amicacid linkages with either the amine or the anhydride and which iscapable of further reaction to increase the molecular weight of eitherthe polyamic acid or the PIBO formed therefrom under conditions otherthan those used to react the aromatic diamine and aromatic dianhydrideto form the polyimide precursor.

Examples of such bifunctional chain extenders include compounds havingone reactive group which is either a primary or secondary amine moiety(i.e., --NH₂ or --NRH), or a cyclicdicarboxylic anhydride moiety and onelatent moiety which is capable of reacting to extend chain length orcross-link the polyamic acid or PIBO such as an unsaturated group(i.e., >C═C< or --C.tbd.C--) or a strained aromatic ring such as acyclobutene which, upon the application of heat will open to furtherreact. Epoxy and isocyanate groups are not suitable as latent moietiessince they generally will react with the amine of the aromaticdiaminobenzoxazole at the conditions normally used in preparing thepolyimide precursor.

Preferred examples of bifunctional chain extenders are represented bythe structural formulas: ##STR29## wherein M is a divalent organicradical, preferably alkylene or arylene, more preferably an alkylenehaving from 1 to about 12 carbon atoms or phenylene, and most preferablymethylene or phenylene; and R is hydrogen, alkyl or aryl, preferablyhydrogen, methyl, or phenyl; and most preferably hydrogen or phenyl.

Representative examples of bifunctional chain extenders include maleicanhydride, nadic anhydride, vinyl phthalic anhydride, 1,2-dimethylmaleicanhydride (which compounds have an anhydride group reactive with theamine and an unsaturation for chain extension);3-(3-phenylethynlphenoxy) aniline, phenylethynlaniline, ethynylaniline,propargyl amine (which compounds have an amino group reactive with theanhydride and an ethylenic or acetylenic unsaturation for chainextension); and benzocyclobutene or other aromatic cyclobutenefunctionalized with an amine or anhydride reactive group such as4-aminobenzocyclobutene (which compounds have an anhydride reactiveamine group and cyclobutene ring, a strained aromatic ring). Preferredbifunctional chain extenders include maleic anhydride,4-aminobenzocyclobutene, nadic anhydride, propargyl amine, andphenylethynlaniline.

The polyimide precursor product will vary depending on the type andamount of each of the aromatic dianhydride, diaminobenzoxazole andbifunctional chain extender reactants employed, and the type and amountof reactants are chosen accordingly. More specifically, the amount ofthe bifunctional chain extender is generally selected to prepare apolyimide precursor of desired molecular weight and solution viscosity.In general, as the amount of the bifunctional chain extender employedincreases, the molecular weight of the resulting product is reduced and,hence, its solution viscosity is also reduced at the same solids level.In general, the desired solution viscosity will vary depending on avariety of factors including the desired end use. For example, when asolution of the polyimide precursor is to used in spin-coatingoperations, the solution will advantageously exhibit a viscosity fromabout 100 to about 10,000 centipoise (cps) as measured using aBrookfield viscometer, Model DV-II+, at 25° C., using Spindle no. 40 ata speed in the range of from 1 to about 10 rpm. Alternatively, in a filmcasting operation, the solution will advantageously exhibit a viscosityfrom about 500 to about 250,000 cps.

In general, the aromatic dianhydride and diaminobenzoxazole areadvantageously employed in amounts from about 0.5 to about 1.5,preferably from about 0.75 to about 1.25, more preferably from about 0.8to about 1.2, equivalents of the aromatic dianhydride per equivalent ofthe diaminobenzoxazole. In general, the bifunctional chain extender isadded in an amount of from about 0.5 to about 0.0004, preferably fromabout 0.3 to about 0.02, more preferably from about 0.1 to about 0.025,equivalents of bifunctional chain extender per equivalent of thearomatic dianhydride or diaminobenzoxazole reactive with the chainextender. Most preferably, the reactants, including thediaminobenzoxazole, aromatic dianhydride, and bifunctional chainextender, are employed in amounts such that the reaction mixturecontains equivalent amounts of amine and anhydride with thediaminobenzoxazole and the aromatic dianhydride both being considered tohave two equivalents.

While not being bound by theory, the reaction of a diamine anddianhydride to form polyamic acid precursor can be represented by usingan aminobenzocyclobutene chain extender: ##STR30## wherein M is anaromatic-containing group such as depicted hereinbefore, n is an integerwith an average from about 2 to about 1,000, more generally from about10 to about 200, and it being understood that the amic acid linkages canexist as a variety of substituted isomers.

It is not particularly critical how the aromatic dianhydride, aromaticdiamine and bifunctional chain extender are contacted, and in general,they are dissolved in a suitable organic liquid as a reaction medium andreacted to form a solution of the desired polyimide precursor. By theterm "polyimide precursor" is meant a polyamic acid having latentreactive end groups which are capable of further reaction to form ahigher molecular weight or cross-linked polymer. The organic liquidreaction medium is preferably a liquid in which the monomers and theresulting polyimide precursor are soluble and is sufficiently inert tothe monomer(s) that it does not significantly and deleteriously affectpreparation of the polyimide precursor. Typically, the organic liquidreaction medium comprises a polar, aprotic liquid, including mixtures ofone or more polar aprotic liquids. While the polar, aprotic liquid(s)most advantageously employed as the reaction medium is dependent on thespecific dianhydride, diaminobenzoxazole, and chain extender employed,as well as the desired reaction product; preferred polar, aproticsolvents are generally amide solvents such as N,N'-dimethylacetamide,N-methylpyrrolidinone, 1,3-dimethyl-5-imidazolidinone,N,N-dimethylformamide, 1,1,3,3-tetramethylurea, orN-cyclohexylpyrrolidinone.

To facilitate the formation of a solution of the monomers or theresulting polymers in the solvent, inorganic salts such as lithiumchloride and/or calcium chloride, can be added to the reaction mixture.However, this practice is not normally preferred. In addition, theorganic liquid reaction medium can also comprise some amounts of anon-reactive, non-solvent for the polyamic acid precursor (e.g. toluene,tetrahydrofuran, 1,4-dioxane and the like) miscible with thenon-reactive polar aprotic liquid and may be used in combination withthe polar aprotic liquid. In general, the non-solvent liquid can beemployed in amounts of up to about 0.75, preferably up to about 0.65,volume fraction; said volume fraction being based on the total volume ofthe polar aprotic liquid and miscible non-solvent.

In one method for preparing the polyimide precursor, the aromaticdianhydride, aromatic diamine, and bifunctional chain extender can beadded separately, either neat or in solution, to a liquid in which thedianhydride, diamine, and chain extender are sufficiently soluble andthen reacted. Alternatively, the organic liquid reaction diluent can beadded to a neat mixture of the monomers or the monomer and polymer; or asolution of either the aromatic dianhydride, aromatic diamine, orbifunctional chain extender can be prepared and the other reactants,either neat or dissolved in a solvent, can be added to this solution andthen reacted. Preferably, the monomers and chain extender, either neator as a solution, more preferably neat, are simultaneously added to theliquid reaction medium.

The reaction of the dianhydride, diaminobenzoxazole, chain extender, andother monomers, if any, can be conducted at any temperature between thefreezing point and the boiling point of the organic liquid reactionmedium, but within that constraint, is preferably conducted at atemperature from about -40° C. to about 150° C., more preferably fromabout -20° C. to about 100° C., and most preferably from about 0° C. toabout 50° C. Due to the equilibrium between amic acid linkage, themonomers' reactive functionalities and the presence of water (generatedfrom thermal imidization) and its hydrolysis of amic acid linkages; thepolymerization, as well as subsequent storage, is most preferablyconducted at a temperature below about 50° C.

The concentrations at which the monomers and chain extender are mostadvantageously employed in the organic liquid reaction medium aretypically limited by the solubility of the monomers and the solubilityand viscosity of the resulting polyimide precursor. While these amountsare dependent on a variety of factors including the specific monomers,chain extender and organic liquid employed and the resulting polymer,the monomers are preferably used in an amount such that theirconcentration in the organic liquid reaction medium is at least about0.1 weight percent. In general, the monomers are employed in an amountfrom about 0.5 to about 75 percent based on the total weight of themonomers and the organic liquid reaction medium. In general, themonomers are preferably employed in an amount from about 1 to about 50,more preferably in an amount from about 2 to about 30 weight percentbased on the total weight of the monomers and organic liquid reactionmedium employed.

In preparing the polyimide precursor, it is most preferred that themonomers, organic liquid reaction medium, and reactor vessel contain aslittle water as possible prior to reaction. In addition, conditions ofextreme cleanliness are also most preferred. Preferably, there shouldinitially be less than 5 mole percent water based on the total moles ofdianhydride, diaminobenzoxazole, chain extender and any optionallyemployed additional comonomers. If necessary, toluene, or othermaterials capable of forming an azeotrope with water, can be added tothe solvent and later distilled off to remove water. The polyimideprecursor is also preferably prepared and stored in both an oxygen-freeand water-free atmosphere. In addition, the monomers can berecrystallized and/or sublimed prior to use to reduce impurities.

In addition to the aromatic dianhydride and aromatic diaminobenzoxazole,the polyimide precursor may optionally be prepared using one or moreother monomers, for example, a non-benzoxazole containing diamine suchas 1,4-phenylenediamine or 4,4'-oxydianiline or 3,4'-oxydianiline.

These optionally employed monomers may be added to the reaction mixtureprior to or during the reaction of the aromatic dianhydride anddiaminobenzoxazole. In general, when employed, the monomers are employedin an amount from about 0.01 to about 50, preferably from about 0.1 toabout 20, more preferably in an amount from about 0.2 to about 5, molepercent based on the total moles of aromatic dianhydride anddiaminobenzoxazole employed.

A compound which has either a single functional group or a compoundhaving only one functional group may also optionally be employed inpreparing the polyamic acid; in which case the compound terminates oneend of the polyamic acid chain without providing sites for chainextension.

Following preparation of the polyimide precursor, the precursor,generally without subsequent isolation, is exposed to conditions toprepare a polyimidebenzoxazole (PIBO) from the polyimide precursor whichis generally accompanied by increase in molecular weight. Any method bywhich a PIBO can be formed from the polyimide precursor can be employedin the practice of the present invention. In general, the polyimideprecursor is simply heated to a temperature sufficient to cause theclosure of amic acid linkages thereby forming PIBO from the polyimideprecursor. The ring-closing reaction will form water and the totalamounts of water generated increases as the reaction proceeds. Althoughthe temperatures at which the desired PIBO can be prepared will varydepending on the latent moiety and whether a chemical ring-closing agentis employed, in general, temperatures from about 100° C. to about 600°C. are advantageously employed when a chemical ring-closing agent is notemployed and from about -10° C. to about 600° C. when such an agent isemployed. At these temperatures, the reaction time required to convertthe polyamic acid to the desired PIBO product can range from as littleas about 5 minutes or less to as much as about 200 hours or more. Ingeneral, the temperatures are more advantageously in a range from about100° C. to about 450° C., preferably from about 150° C. to about 400° C.

Selection of specific reaction temperatures and times are dependent upona number of factors including the composition of the precursor, theorganic liquid(s) used as the reaction medium, and concentration ofprecursor in the reaction medium and the desired PIBO product. Ingeneral, the PIBO is advantageously prepared by exposing the polyimideprecursor to an initial conversion (imidization) temperature of at leastabout 150° C. Conversion temperatures from about 160° C. to about 280°C. are more preferred and from 185° C. to about 230° C. most preferred.In general, these conversion temperatures are maintained for at leastabout 5 and no more than about 90, preferably from about 10 to about 80,more preferably from about 15 to about 75, minutes.

In a preferred method of the present invention, at least a portion ofthe organic liquid(s) used as the reaction medium is removed from thepolyimide prescursor and the precursor then subjected to the conversiontemperature. Some ring-closure or chain extension may also occur duringremoval of the organic liquid. To remove the necessary amounts of theorganic liquid from the solution of the polyimide precursor, it can becontacted with amounts of a non-solvent for the polyamic acid such aswater, methanol, acetone or other liquid and at conditions to coagulatethe polyimide precursor; heated to a temperature to volatilize thesolvent; or a combination of both.

In general, sufficient amounts of the organic liquid are removed fromthe polyimide precursor solution such that less than about 60 percentremains based on the weight of the organic liquid and the polyimideprecursor. Preferably, sufficient amounts of organic liquid are removedsuch that there is less than about 45, more preferably less than about35, percent organic liquid remaining based on the weight of the organicliquid and polyimide precursor. The polyimide precursor typically bindssome organic liquid until imidization; thereby making it difficult toreduce the solvent level by volatilization to below about 10 weightpercent.

In general, the organic liquid is vaporized at a temperature from about50° C. to about 130° C., with the vaporization temperature(s) mostadvantageously employed within this range being dependent on a number offactors, particularly the vapor pressure/boiling point of the specificorganic liquid(s) employed as the reaction medium. For example, atemperature of at least 50° C. is preferred when the organic liquidreaction medium comprises N,N'-dimethylacetamide and a temperature of atleast about 80° C. is preferred when the organic liquid reaction mediumcomprises N-methylpyrrolidinone. The time to which the precursorsolution is heated at the vaporization temperature is dependent on avariety of factors including the organic liquid reaction medium, percentsolids, solution viscosity, exhaust rate, and the presence or absence ofa coagulating fluid and chemical ring-closing agents, but, in general, aminimum of about 5 minutes is required with a maximum normally beingless than about two hours. In many applications, the precursor solutionis kept at the vaporization temperature until it resembles more of asolid, shaped article than a polymer solution.

The polyamic acid is then heated to the conversion temperature. Ingeneral, the temperature is increased from the vaporization temperatureto the conversion temperature as quickly as possible. Preferably, thetransition from an environment at the vaporization temperature to anenvironment at the conversion temperature should take less than about30, preferably no more than about 10, and most preferably no more thanabout 5 minutes.

In the practice of the present invention, following exposure to theconversion temperature, to further improve PIBO properties, the reactionproduct is preferably further treated by exposure to a temperature fromabout 250° C., preferably from about 300° C., to a temperature which isless than about 600° C. and less than the glass-transition temperatureof the resulting PIBO, for at least about 10 seconds. Preferably, theproduct is maintained at this elevated temperature for a sufficient timeto insure sufficient conversion of the amic acid and/or isoimidelinkages into imide linkages to obtain the desired PIBO properties. Thisfurther heat-treatment can unexpectedly increase certain film propertiessuch as tensile strength and/or elongation-to-break. In one method, thePA/PIBO product is exposed to sequentially increasing heat-treatingtemperatures below the glass transition temperature. In a most preferredembodiment, following exposure to the conversion temperature, thePA/PIBO is exposed to (i) a single temperature of greater than about350° C. and less than about 600° C. for between 0.1 to 120 minutes or,alternatively, to (ii) an annealing temperature from about 250° C. toabout 400° C. for from about 0.1 to about 120 minutes and thenoptionally to a heat-treating temperature, higher than the annealingtemperature, from about 260° C. to about 600° C. for from about 0.1 toabout 120 minutes. The reaction(s) to prepare the PIBO from theprecursor are preferably conducted in an atmosphere of air or an inertgas that is preferably moisture free.

In preparing PIBO articles, if the PIBO product is not soluble in theorganic liquid(s) employed as the reaction medium, the precursorsolution is formed into the desired shape of the finished product suchas a film, fiber or other article using conventional techniques such asextrusion before substantial conversion of the polyamic acid to PIBO,i.e., either before or during the removal of organic liquid used as thereaction medium. If the PIBO product is soluble in the organic liquid(s)employed as the reaction medium, it is not normally necessary and maynot be desirable to preshape the precursor solution. While the precursorcan be removed from solution and redissolved in another solvent, thepolyamic acid is preferably processed directly from the organic liquidreaction solution (diluted, if necessary, with additional amounts of thesame or compatible solvent).

Shaping the precursor solution to form a film can be done by coating,casting, dipping, or spraying the precursor solution onto an appropriatesubstrate and imidizing the polyamic acid either partially orcompletely. The film may be removed either before or after theimidization reaction. Films as thin as 1 micrometer, preferably having athickness from about 1 to about 2000, micrometers, can be made.Appropriate substrates include materials such as glass, aluminum,stainless steel, silicon, copper, polyimide film and eventetrafluoroethylene fluorocarbon polymers (marketed under the tradenameTeflon™) to form a supported film. Often, a release agent such asFluoroglide™ or IMS Silicone Spray No. S316 are used to facilitateremoval of the film from the substrate.

Alternatively, the PIBO can permanently coat these or other substrates,either as a single layer coating or as a coating applied in multiplelayers. The precursor solution can be applied at essentially any coatingthickness and relatively thick coatings of up to 2000 micrometers can beprepared. While the desired thickness of the PIBO coating depends on theend use application, it will generally vary from about 0.1 to about 100micrometers. The coated substrates can be prepared using a variety oftechniques well-known in the art for coating liquid solutions on asubstrate and include spraying, or spin- or dip-coating the precursorsolution onto a clean substrate. To improve the adhesion of the PIBO tothe substrate, an adhesion promoter such as 3-aminopropyltriethoxysilanecan optionally be employed.

The thickness at which the precursor solution is applied as coating hasa dependence on the viscosity of the solution. In general, for any givenprecursor, the coating thickness is reduced as the viscosity of thesolution is reduced. The viscosity most preferably employed will dependon a variety of factors including the desired thickness of the coatingand the coating techniques. For example, using spin-coating techniquesto prepare coatings of less than 100 micrometer, the viscosity of thepolyamic acid solution (as measured by a Brookfield viscometer at 25°C.) is preferably less than about 15,000 centipoise (cps), morepreferably less than about 10,000 cps. Viscosity reduction can beachieved by simply diluting the polyamic acid solution to a desiredviscosity.

One method for preparing multiple layered PIBO coatings involvesapplying a precursor solution, volatilizing the organic liquid(s) usedas the reaction medium, applying another layer of precursor solution andvolatilizing the organic liquid(s) used as the reaction medium, andrepeating this process until such time as the desired thickness isobtained. The polyamic acid is then converted to PIBO. Alternatively, amultilayer coating can be prepared using a step-wise technique ofrepeated application and imidization of individual polyamic acid layersuntil the desired level of coating thickness has been applied. Anoptional adhesion promoter can be applied between layers. These types ofthin PIBO coatings are useful for electrical insulating purposes andenvironmental resistance.

While the reaction of the polyamic acid to form PIBO is effectivelyconducted by merely heating the polyamic acid, imidization can beconducted in the presence of a material or combination of materialswhich facilitate or accelerate ring-closing (e.g., converts the amicacid linkages into imide linkages by catalyzing ring-closure viadehydration). Of these materials, those whose by-products can be removedafter or during imidization by vaporization and/or washing are mostpreferred. Materials which can be employed include dehydration agentssuch as acetic anhydride and other materials listed in columns 5 and 6of U.S. Pat. No. 3,410,826, which is incorporated herein by reference;with the preferred materials being acetic anhydride, propionicanhydride, ketene, and isobutyric dianhydride. While these materials canbe employed alone, they are most preferably employed with an organicbase, preferably those which do not form inorganic salts, whichfacilitate ring-closing activity such as picoline, pyridine,isoquinoline, triethylamine, lutidine or their mixtures, and othermaterials listed in column 6 of U.S. Pat. No. 3,410,826. Thesering-closing agents (i.e., the dehydrating agent and organic base) aregenerally mixed together before use although they can be addedseparately to the precursor solution.

While the amount of ring-closing agent most advantageously employed isdependent on a variety of factors including the specific ring-closingagent(s) employed; the desired reaction times and temperatures and thePIBO being prepared, the dehydration agent is preferably employed in anamount of more than about 10, more preferably more than about 100, andmost preferably at least about 200, mole percent, preferably being usedin an amount of less than about 500, more preferably less than about400, mole percent, said mole percent being based on the moles ofdehydration agent per mole of amic acid linkage. The organic base isemployed in an amount of about 1 to about 200, preferably from about 10to about 150, more preferably from about 20 to about 100, mole percentbased on the theoretical maximum number of moles of amic acid linkages.The theoretical maximum number of polyamic acid linkages is easilydetermined by the moles of dianhydride employed.

These ring-closing agents may by added to the precursor solution beforeor during removal of the organic liquid(s) used as the reaction mediumor subsequent reaction. The temperature at which the precursor solutionand ring-closing agents are mixed is advantageously from about -20° C.up to about 140° C., more preferably from about -20° C. to about 50° C.,most preferably from about -20° C. to about 15° C. to minimize gelationor rapid buildup of viscosity of the polyamic acid solution. While thering-closing agents, if any, can be added neat to the polyamic acidsolution they are preferably added as a solution, preferably in asolution of from about 5 to about 50, weight percent, in an organicliquid which is compatible with the polyamic acid solution. Preferably,the solution of the ring-closing agents is prepared using the samesolvent as employed in the polyamic acid solution.

The precursor solution can first be formed into a desired shape and theshaped article contacted with a solution of the ring-closing agents,generally for a time of at least about 30 seconds, but less than about30 minutes. In general, once the precursor solution has been contactedwith ring-closing agent, conversion of amic acid linkages to imidelinkages begins, with the rate of conversion being temperaturedependent.

In preparing PIBO articles, additives such as fillers, pigments, carbonblack, conductive metal particles, abrasives and lubricating polymersare often advantageously employed. Conventional additives can beemployed and the method of incorporating the additives is not critical.They can conveniently be added to the precursor solution prior topreparing the shaped article. The precursor solution, alone orcontaining additives, may be applied by any of the usual techniques(doctoring, rolling, dipping, brushing, or spraying) onto a number ofdifferent substrates. If the PIBO polymer is prepared in solid form, theadditives can be added to the solution prior to processing into a shapedarticle.

For PIBO which remains soluble in the organic liquid reaction medium, itis generally preferred to add the ring-closing agents directly to theprecursor solution, allow the reaction to proceed at slightly elevatedtemperatures, e.g., up to 50° C., but most preferably room temperature(about 20° C. to about 25° C.), followed by volatilization of thesolvent and heat-treatment. In general, when the resulting PIBO issoluble, ring-closing agents are preferably employed.

The following examples are presented to illustrate the invention andshould not be construed to limit its scope. In the examples, all partsand percentages are by weight unless otherwise indicated.

The molecular weight of the polyimide precursor can be expressed, albeitindirectly, in terms of its inherent viscosity. Inherent viscosity("IV") is expressed as:

    η.sub.inh =1n (η.sub.rel)/c

wherein η_(rel) is the relative viscosity or t/t_(o) where t is the flowtime for the polyimide precursor solution and t_(o) is the flow time forthe pure solvent, and c is concentration of polyimide precursor insolution, given in grams per deciliter. The units for IV are decilitersper gram ("dL/g"). An increase in molecular weight is generallyaccompanied by an increase in solution viscosity.

In these examples, inherent viscosities are measured by transferring analiquot of the solution to a 25.0 mL volumetric flask and diluting withN-methyl-pyrrolidinone to achieve a solution concentration of about 0.2g/dL. Solvent and solution flow times are measured at 25.0° C. using aSchott-Gerate CT 1450/AVS 310 viscometer in a Ubbelohde viscometer tubewith an inner diameter of approximately 0.63 mm.

Tensile properties are measured according to ASTM D882. Adhesion ismeasured using a Cross-Hatch Tape Peel Adhesion Test. In such test, acoated wafer is scored using a surgical blade in a manner which produces100 squares approximately 1 millimeter by 1 millimeter (mm). The coatingis then attempted to be removed from the wafer using Scotch 810 tape andScotch 600 tape. Four peels, two with each type tape are conducted. Theresults are scored as number of squares pulled off by the tape per 100squares. The lower the number, the better the adhesion of the film. Thetest is then repeated after the wafers are immersed in a 95° C. waterbath for one hour and cooled to room temperature.

EXAMPLE 1

Into a 3-neck, 500-mL round-bottom flask equipped with agitation meansand a Dean-Stark trap with a condenser is fed 230 milliliters (mL) ofN,N'-dimethylacetamide (DMAC) and 100 mL of toluene. The flask is gentlypurged with nitrogen and the toluene then distilled off. Into thestirred, room temperature, solvent in the flask is added 20 mL dry DMAC;17.729 gram (g) of 2,6-(4,4'-diaminophenyl)benzo 1,2-d:5,4-d'!bisoxazole(51.446 mmol) containing 0.70 weight percent of N-methylpyrrolidinone(NMP); 15.695 g of 4,4'-oxydiphthalic anhydride (50.593 mmol), and 0.167g of maleic anhydride (1.7 mmol). After about 68 hours, the resultingpolyimide precursor has an inherent viscosity (IV), measured in NMP, at25.0° C., and 0.2 g/dL, of 1.40 dL/g.

The resulting 12.5 percent solution of polyimide precursor end-cappedwith maleic anhydride is spin-cast onto a clean 6" bare silicon wafer ata spread cycle of 38 seconds and 500 revolutions per minute (rpm) and aspin-cycle of 30 seconds at 6000 rpm. The coated wafer is then heated ina nitrogen atmosphere maintained at 27 minutes at 30° C., heated for 15minutes to 60° C., maintained at 60° C. for 15 minutes, heated for 30minutes to 225° C. and maintained at 225° C. for 15 minutes, and thenheated to 300° C. for 20 minutes and maintained at 300° C. for 1 hour.The wafer is then cooled to room temperature. The film is scored with arazor blade and removed from the wafer. The film thickness is measuredto be 2.7 microns (μm). The procedure is repeated and the film thicknessis measured to be 2.9 μm. The procedure is repeated twice more exceptthat the spin cycle in the spin-coating operation is 30 seconds at 7000rpm. The film thickness of the two samples prepared in this manner is2.6 μm and 2.5 μm, respectively. The film exhibits a tensile strength of37.3 Ksi, a tensile modulus of 1.28 Msi, and an elongation of 16.2percent.

An oxidized 4" silicon wafer with aluminum structures is prepared withthe height of the aluminum structures on the wafer approximately 1.8microns, with a width varying between 18 microns and 1000 microns.Before spin-casting of the precursor solution, the thus prepared waferis Plasma-cleaned with oxygen for 15 minutes, and is dump-rinsed withwater 3 times. The wafer is then spin-dried in air. An adhesion promoterof 3-aminopropyltriethyoxy silane is then spin-coated on the wafer. Theprecursor solution is then spin-coated on the wafer using a spread cycleof 500 rpm for 3 seconds and a spin-cycle of 1500 rpm for 30 seconds.

The polyimide precursor is then cured into a PIBO film by heating thecoated wafer in a nitrogen atmosphere maintained at 27 minutes at 30°C., heated for 15 minutes to 60° C., maintained at 60° C. for 15minutes, heated for 30 minutes to 225° C. and maintained at 225° C. for15 minutes, and then heated to 300° C. for 20 minutes and maintained at300° C. for 1 hour. The wafer is then cooled to room temperature. Thefilm thickness is measured to be 9.23 μm.

The profile of the aluminum structures and the degree of planarizationor planarization ratio of the PIBO on the wafer is measured by using aprofilometer. The results are summarized in the following table.

    ______________________________________                                        Al Width  Al Step      Film Step Planarization                                Bottom/top (μm)                                                                      Height (μm)                                                                             Height (μm)                                                                          Ratio                                        ______________________________________                                        18/2      0.910        0.320     0.648                                        18/2      1.010        0.48      0.525                                        24/2      1.705        0.89      0.478                                         46/20    1.825        1.475     0.192                                        114/90    1.825        1.585     0.132                                         330/304  1.820        1.625     0.107                                        1010/990  1.815        1.695     0.066                                        ______________________________________                                    

EXAMPLE 2

Using the techniques of Example 1, a polyimide precursor is preparedusing an initial mixture of 59.9 mL of N-methylpyrrolidinone (NMP) and25 mL of toluene (the toluene being distilled off) and a monomer chargeof 9.185 g of 5-amino-2-(p-aminophenyl)-benzoxazole (p-DAMBO, 40.78mmol), 0.286 g of maleic anhydride (2.91 mmol), and 8.577 g ofpyromellitic dianhydride (PMDA, 39.32 mmol). The residual monomers arerinsed down using 10 mL of dry NMP. After 44 hours, the resultingpolyamic acid precursor has an inherent viscosity (IV) measured in NMP,at 25.0° C., and 0.2 g/dL, of 1.19 dL/g.

The flask is chilled with ice for 10 minutes, then degassed using anaspirator vacuum for 45 minutes while chilled. The flask is brought backto atmospheric pressure using nitrogen and a mixture of 12.2 mL of NMP,8.13 mL of acetic anhydride (AA, 86.2 mmol), and 2.16 mL of 3-picoline(3-P, 22.2 mmol) are added dropwise to the stirred, chilled polyamicacid solution over 3 minutes. The solution is stirred for 5 minutes,then degassed using an aspirator vacuum for 17 minutes. The flask isbrought back to atmospheric pressure with nitrogen.

The resulting solution (about 17.5 weight percent solution of p-DAMBOand PMDA polyamic acid capped with MA in NMP) is cast onto a clean glassplate with a doctor blade with a 0.015" gap. The film is heated in anair-circulating oven at 100° C. for 40 minutes. The film is removed fromthe glass and secured in an aluminum frame. The film is then heated in anitrogen atmosphere maintained at 27 minutes at 30° C., heated for 30minutes to 225° C., maintained at 225° C. for 30 minutes, heated for 20minutes to 300° C. and maintained at 300° C. for 120 minutes, and thencooled to ambient temperature.

The film sample exhibits a tensile strength of 35.25 Ksi, a tensilemodulus of 1.41 Msi, and an elongation of 4.5 percent.

EXAMPLE 3

A polyimide precursor is prepared using the same techniques as used inExample 2, except that the initial monomer feed consists of 9.180 g ofp-DAMBO (40.76 mmol), 0.307 g of MA (3.14 mmol), and 8.548 g of PMDA(39.19 mmol); the resulting precursor solution is diluted with 8.4 mL ofNMP; and is reacted with a mixture 20.0 mL of NMP, 8.11 mL of AA (86.0mmol) and 2.15 mL of 3-P (22.1 mmol) prior to being cast as a film andcured. The polyimide precursor has an inherent viscosity (IV), measuredin NMP, at 25.0° C., and 0.2 g/dL, of 1.29 dL/g.

The cured film exhibits a tensile strength of 36.60 Ksi, a tensilemodulus of 1.42 Msi, and an elongation of 5.6 percent.

EXAMPLE 4

Using the techniques of Example 1, a polyimide precursor is preparedfrom 17.420 g of p-DAMBO (77.336 mmol), 2.304 g of aminobenzocyclobutene(19.33 mmol); and 18.977 g of PMDA (87.003 mmol) in a solvent liquidreaction diluent of 149.9 mL of dry NMP (no toluene is employed andthere is no distillation step). After 44 hours reaction time, the bulksolution viscosity is 3705 centipoise at 25.0° C. at 5.0 rpm on aBrookfield Model DV-11+ viscometer using spindle cp40. At this time, theresulting polyimide precursor has an inherent viscosity (IV) measured inNMP, at 25.0° C., and 0.2 g/dL of 0.56 dL/g.

An oxidized 4" silicon wafer with aluminum structures is prepared by astandard procedure. The height of the aluminum structures on the waferare about 1.9 microns, and their width varies between 3.3 microns and1000 microns. The wafer is Plasma-cleaned with oxygen for 15 minutes,dump-rinsed with water 3 times, and then spin-dried in air. An adhesionpromoter is then spin-coated onto the wafer. The precursor solution isthen spin-coated on the wafer using a spread cycle of 500 rpm for 10seconds and a spin cycle of 3900 rpm for 30 seconds in a spin-caster.The coated wafer is exposed to the same heating schedule as Example 1except that the initial temperature is 80° C. rather than 60° C.

The thickness of the PIBO film on the wafer is 3.8 microns. The profileof the aluminum structures and the degree of planarization, orplanarization ratio of the PIBO on the structures, are measured using aprofilometer. The results are summarized in the following table.

    ______________________________________                                        Al Width Al Step      Film Step Planarization                                 (μm)  Height (μm)                                                                             Height (μm)                                                                          Ratio                                         ______________________________________                                         4       1.850        0.915     0.505                                         11       1.870        1.270     0.321                                         32       1.875        1.390     0.259                                         100      1.875        1.505     0.197                                         320      1.875        1.740     0.072                                         1000     1.860        1.805     0.030                                         ______________________________________                                    

EXAMPLE 5

Using the same techniques as Example 4, a polyimide precursor solutionis prepared from 19.400 g of p-DAMBO (86.127 mmol), 3.378 g of MA,(34.45 mmol), 15.029 g of PMDA (68.901 mmol), in 104.2 mL of dry NMP.The resulting precursor has an inherent viscosity (IV) measured in NMP,at 25.0° C., and 0.2 g/dL, of 0.309 dL/g.

Using the same techniques as Example 4, the precursor is used to coat anoxidized 4" silicon wafer (with the height of the aluminum structuresbeing about 1.9 microns, and the width varying between 3.3 microns and1000 microns) using a spread cycle of 500 rpm for 15 seconds and a spincycle of 4600 rpm for 30 seconds in a spin-caster. The coated wafer isthen heated in a nitrogen atmosphere maintained at 27 minutes at 30° C.,heated for 15 minutes to 80° C., maintained at 80° C. for 15 minutes,heated for 30 minutes to 225° C. and maintained at 225° C. for 15minutes, and then heated to 300° C. for 20 minutes and maintained at300° C. for 1 hour. The wafer is then cooled to room temperature.

The thickness of resulting PIBO coating on the wafer is 4.175 microns.The profile of the aluminum structures and the degree of planarization,or planarization ratio, of the PIBO on the structures are measured byusing a profilometer. The results are summarized in the following table.Small amounts of particulates are observed in the coating.

    ______________________________________                                        Al Width  Al Step Ht.  Film Step Planarization                                Bottom/top (μm)                                                                      Height (μm)                                                                             Height (μm)                                                                          Ratio                                        ______________________________________                                        26/2      1.670        0.815     0.555                                        30/6      1.930        1.260     0.356                                         54/22    1.985        1.525     0.225                                        120/92    1.995        1.720     0.132                                         335/310  2.005        2.010     -0.008                                        1030/1000                                                                              2.010        2.030     -0.014                                       ______________________________________                                    

EXAMPLE 6

Using the method of Example 1, a polyimide precursor is made from 2.320g of p-DAMBO (10.30 mmol), 0.040 g of MA (0.40 mmol), and 2.203 g ofPMDA (10.10 mmol) in 25.0 mL of N-methyl pyrrolidinone (dried with 10 mLof toluene which has been distilled off) exhibits an inherent viscosity(IV) measured in NMP, at 25.0° C., and 0.2 g/dL, of 1.80 dL/g. Its bulksolution viscosity is 125,950 centipoise measured on a Brookfield ModelDV-11+ Viscometer at 25.0° C. at 1.0 rpm with spindle cp51. After beingcast onto a clean glass plate with a doctor blade with a 0.030" gap, thefilm is heated in an air-circulating oven at 100° C. for 60 minutes andremoved from the oven. The oven is warmed to 225° C. and the supportedfilm is placed in the oven for about 10 minutes with film pulling fromthe glass and breaking. The brittle film has a thickness of about 2 milswhich for this specific polymer and these processing conditions were toothick for practical use.

EXAMPLE 7

Using the techniques of Example 2, a polyimide precursor is preparedfrom 13.853 g of p-DAMBO (61.500 mmol), 0.119 g of MA (1.22 mmol), and13.282 g of PMDA (60.892 mmol) in 149.5 mL NMP (dried with 20 mL oftoluene which has been distilled off). The resulting precursor has aninherent viscosity (IV) measured in NMP, at 25.0° C., and 0.2 gldL, of1.80 dL/g. The bulk solution viscosity is 105,900 centipoise measured ona Brookfield Model DV-II+ Viscometer at 25.0° C. at 1.0 rpm with spindlecp51.

Using techniques of Example 2, the resulting precursor solution isreacted with 12.2 mL of acetic anhydride (0.129 mol) and 3.2 mL of3-picoline (33 mmol) which are added dropwise as a solution in 35.2 mLof NMP.

The solution is cast onto a clean glass plate with a doctor blade with a0.040" gap and heated in an air-circulating oven at 80° C. for 90minutes and 100° C. for 30 minutes with a 90° rotation every 30 minutes.The film is removed from the oven and framed. The oven is warmed to 225°C. and the film placed in the oven for about 50 minutes. The oventemperature is then raised to 300° C. over 20 minutes and held at 300°C. for 2 hours. The framed film is then heated, in a nitrogen atmosphereto 400° C. for 30 minutes and maintained at 400° C. for 2 hours. Thefilm is then cooled to room temperature. The film has a thickness of49.8 μm. The film exhibits tensile strength of about 55.65 Ksi, atensile modulus of about 1.28 Msi, and an elongation of about 19.2%.

EXAMPLE 8

A 3-neck, 250 mL round-bottom flask equipped with agitation means is fedwith 20.423 g of 5-amino-2-(m-aminophenyl)benzoxazole (90.669 mmol),2.540 g of maleic anhydride (25.91 mmol), 16.952 g of pyromelliticdianhydride (77.716 mmol), and 90.0 mL of dry NMP. The flask is gentlypurged with nitrogen. The mixture is further diluted with 9.4 mL of dryNMP. After 44 hours, the resulting polyamic acid precursor has aninherent viscosity (IV) measured in NMP, at 25.0° C., and 0.2 g/dL, of0.340 dL/g. The solution is further diluted by adding 7.7 mL of dry NMP.The viscosity of the resulting solution is measured to be 5876centipoise on a Brookfield Model DV-II+ viscometer at 25.0° C., spindlecp40, at 2.5 rpm.

An oxidized 4" silicon wafer with aluminum structures having a height ofapproximately 1.9 microns, and widths which vary between 3.3 microns and1000 microns is plasma-cleaned with oxygen for 15 minutes anddump-rinsed with water 3 times. The wafer is then spin-dried in air andthen an adhesion promoter is spin-coated on the wafer. The polyamic acidsolution is spin-coated on the wafer using a spread cycle of 500 rpm for10 seconds and a spin-cycle of 5950 rpm for 30 seconds in a spin-caster.The PAA coated wafer is then converted to PIBO by heating the coatedwafer in a nitrogen atmosphere maintained for 27 minutes at 30° C.,heated for 15 minutes to 80° C., maintained at 80° C. for 15 minutes,heated for 30 minutes to 225° C., maintained at 225° C. for 15 minutes,heated for 20 minutes to 300° C. and maintained at 300° C. for 60minutes, and then cooled to ambient temperature.

The thickness of resulting PIBO coating is about 4.085 microns. Theprofile of the aluminum structures and the degree of planarization, orplanarization ratio, of the PIBO on the structures are measured using aprofilometer. The results are summarized in the following table. Smallamounts of particulates are observed in the coating.

    ______________________________________                                        Al Width  Al Step      Film Step Planarization                                Bottom/top (μm)                                                                      Height (μm)                                                                             Height (μm)                                                                          Ratio                                        ______________________________________                                        13/1      1.525        0.380     0.793                                        26/2      1.570        0.700     0.585                                        32/4      1.865        1.170     0.373                                         54/22    1.945        1.485     0.236                                        125/90    1.950        1.650     0.149                                         340/305  1.950        1.960     -0.003                                        1030/1000                                                                              1.940        1.925     0.006                                        ______________________________________                                    

EXAMPLE 9

Using techniques similar to those of Example 2, a polyamic acid isprepared from 46.042 g of 2,6-(4,4'-diaminodiphenyl)benzo1,2-d:5,4-d'!bisoxazole (0.13376 mol) containing 0.54 weight percentNMP,0.434 g of maleic anhydride (4.43 mmol), and 40.807 g of4,4'-oxydiphthalic dianhydride (0.13154 mmol) as a solution in 650 mL ofDMAC and 130 mL of toluene (the toluene being distilled off prior tomonomer addition). After about 68 hours, the resulting polyimideprecursor has an inherent viscosity (IV), measured in NMP at 25.0° C.,and 0.2 g/dL of 1.37 dL/g.

Using the techniques of Example 2, a 53.12 g portion of this solution isreacted with 2.01 mL of acetic anhydride and 0.53 mL of 3-picoline(added as a solution in 4.9 mL of DMAC).

The resulting solution is cast onto a clean glass plate with a doctorblade with a 0.010" gap. The film is heated in an air-circulating ovenat 60° C. for 30 minutes and then removed from the glass and secured inan aluminum frame. It is then heated, in the air-circulating oven, to225° C., maintained at 225° C. for 15 minutes, heated for 20 minutes to300° C., maintained for 60 minutes at 300° C. The framed film is thenheated in a nitrogen atmosphere maintained for 27 minutes at 30° C.,heated for 75 minutes to 350° C., and maintained at 350° C. for 120minutes, and then cooled to ambient temperature. The thickness of theresulting film is 8.8 μm.

EXAMPLE 10

Using the techniques of Example 2, a polyimide precursor is preparedfrom 9.866 g of p-DAMBO (43.80 mmol), 0.094 g of MA (0.094 mmol), and9.449 g of PMDA (43.32 mmol) in 100 mL of N,N'-dimethylacetamide (DMAC)and 20 mL of toluene (the toluene being distilled off prior to additionof the monomers and the residual monomers being rinsed with 17.4 mL ofdry DMAC. After 44 hours, the resulting polyamic acid precursor has aninherent viscosity (IV) measured in NMP, at 25.0° C. and 0.2 g/dL, of2.34 dL/g.

The resulting polyimide precursor is reacted with 8.66 mL of aceticanhydride (91.8 mmol), and 2.28 mL of 3-picoline (23.4 mmol) (added toan ice-chilled solution as a solution in 21.0 mL of DMAC). The solutionis stirred for 4.0 minutes with subsequent degassing by aspirator vacuumfor 15.0 minutes. The flask is brought back to atmospheric pressure withnitrogen and the solution cast onto a clean glass plate with a doctorblade (12" width) with a 0.040" gap. The film is heated in anair-circulating oven at 60° C. for 2 hours. The film is removed from theglass and secured in an aluminum frame. The framed film is heated in anair-circulating oven heated at 225° C. for 50 minutes, then heated for20 minutes to 300° C., maintained for 2 hours at 300° C. and then cooledto room temperature. The film is then heat-treated in a nitrogenatmosphere at 400° C. for 2 hours.

The film exhibits a tensile strength of 45.2 Ksi, a tensile modulus of1.36 Msi, and an elongation of 12.6%.

EXAMPLE 11

Using the techniques of Example 2, a polyimide precursor is preparedfrom 142.000 g of p-DAMBO (0.63041 mol), 1.359 g of MA (0.01386 mol),and 135.993 g of PMDA (0.62348 mol) dissolved in 1983 mL of NMP and 100mL of toluene (the toluene being distilled off prior to monomeraddition). After 68 hours, the resulting polyimide precursor has aninherent viscosity (IV) measured in NMP, at 25.0° C., and 0.2 g/dL, of2.39 dL/g and a bulk solution viscosity of 117,800 centipoise asmeasured on a Brookfield Model DV-II+ Viscometer at 1.0 rpm with spindlecp51.

The resulting solution is cast onto a clean glass plate with a 15"doctor blade with a 0.040" gap. The film is heated in an air-circulatingoven at 100° C. for 90 minutes. The film is removed from the glass andsecured in an aluminum frame. The framed film is heated in anair-circulating oven for 30 minutes at 120° C., 10 minutes at 225° C.,10 minutes at 400° C. and then cooled to room temperature. The resultingfilm has a tensile strength of 42.9 Ksi, a tensile modulus of 1.51 Msi,and an elongation of 7.6%.

COMPARATIVE EXAMPLE A

A polyimide precursor is prepared from 3.400 g of p-DAMBO (15.09 mmol),0.407 g of phthalic anhydride (2.74 mmol of a monofunctional end-cappingagent), and 6.096 g of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride (13.72 mmol) with residual monomers rinsed down by 2.4 mL ofdry N-methylpyrrolidinone (NMP) in 20 mL NMP and 10 mL of toluene (thetoluene having been distilled off and the flask being purged withnitrogen prior to monomer addition). After about 22 hours, the resultingpolyimide precursor has an inherent viscosity (IV), measured in NMP, at25.0° C., and 0.2 g/dL, of 0.39 dL/g. The resulting solution is furtherdiluted by adding 6.2 mL of NMP.

The reaction product is reacted with 4.22 mL of acetic anhydride and1.08 mL of 3-picoline (added as a solution in 12.8 mL of NMP). Thismixture is allowed to warm to room temperature and kept at roomtemperature for about 72 hours. The resulting product is precipitatedinto deionized water, rinsed several times with fresh deionized water,collected by filtration, and dried to a constant weight of 8.38 g in a50° C. vacuum oven. The inherent viscosity of the chemically imidizedproduct is 0.34 dL/g when measured in NMP, at 25.0° C.; and 0.2 g/dL.

The resulting isolated solid is redissolved in 21.1 mL of NMP. Thisresulting solution is cast onto a clean silylated Pyrex™ glass platewith a doctor blade with a 0.015" gap. The film is heated in anair-circulating oven at 100° C. for 1 hour. The film is then heated inan air-circulating oven heated to 225° C. for 20 minutes, heated for 20minutes to 300° C., and maintained at 300° C. for 1 hour. The supportedfilm is cooled on its supporting glass and it cracks into small pieceswhile cooling.

COMPARATIVE EXAMPLE B

Using the techniques of Comparative Example A, a polyimide precursor isprepared from 3.400 g of p-DAMBO (15.09 mmol), 0.279 g of phthalicanhydride (1.89 mmol of a monofunctional terminal chain-capping agent),and 6.287 g of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride(14.15 mmol) with residual monomers rinsed down by 2.3 mL of dryN-methylpyrrolidinone (NMP) in 20 mL of NMP and 10 mL of toluene (thetoluene having been distilled off and the flask being purged withnitrogen prior to monomer addition). After about 22 hours, the resultingpolyimide precursor has an inherent viscosity (IV), measured in NMP, at25.0° C., and 0.2 g/dL, of 0.484 dL/g. The resulting solution is dilutedby adding 10.1 mL of NMP.

The resulting precursor is reacted with 4.22 mL of acetic anhydride and1.08 mL of 3-picoline (added to an ice-chilled solution in 12.8 mL ofNMP) at room temperature and kept at room temperature for about 48hours. The product is precipitated into deionized water, rinsed severaltimes with fresh deionized water, collected by filtration, and dried toa constant weight of 8.61 g in a 50° C. vacuum oven. The inherentviscosity of the chemically imidized product is 0.438 dL/g when measuredin NMP, at 25.0° C., and 0.2 g/dL.

The resulting, chemically imidized, product is dissolved with 24.8 mL ofNMP and cast onto a clean silylated Pyrex™ glass plate with a doctorblade with a 0.015" gap. The film is heated in an air-circulating ovenat 100° C. for 1 hour. The film is then heated in an air-circulatingoven heated to 225° C. for 20 minutes, heated for 20 minutes to 300° C.,and maintained at 300° C. for 1 hour. The film is cooled on itssupporting glass and does not crack into pieces. However, the resultingfilm was brittle.

EXAMPLE 12

Into a 3-neck, 2000-mL round-bottom flask equipped with agitation meansand a Dean-Stark trap with a condenser is fed 417.5 mL ofN-methylpyrrolidinone and 30 mL of toluene. The flask is gently purgedwith nitrogen and the toluene then distilled off. After cooling to roomtemperature, 812.5 mL of anhydrous tetrahydrofuran is added to the flaskwith the flask kept under positive nitrogen. Into the stirred, roomtemperature organic liquid reaction medium is added 73.774 g of5-amino-2-(p-aminophenyl)benzoxazole (0.32752 mol); 70.652 g ofpyromellitic dianhydride (0.32391 mol); 0.706 g of maleic anhydride(7.20 mmol); and 20.0 mL of anhydrous N-methylpyrrolidinone. After about68 hours, the resulting polyimide precursor has an inherent viscosity(IV), measured in NMP, at 25.0° C., and 0.2 g/dL, of 2.41 dL/g.

EXAMPLE 13

Into a 3-neck, 100 mL round-bottom flask equipped with agitation meansand a Dean-Stark trap with condenser is fed 22.5 mL ofN-methylpyrrolidinone and 32.5 mL of toluene. The flask is gently purgedwith nitrogen and 10.0 mL of toluene then distilled off. After coolingto room temperature, the flask is kept under positive nitrogen. Into thestirred, room temperature organic liquid reaction medium is added 2.961g of 5-amino-2-(p-aminophenyl)benzoxazole (13.14 mmol); 2.836 g ofpyromellitic dianhydride (13.00 mmol); and 0.028 g of maleic anhydride(0.29 mmol). After about 44 hours, the resulting polyimide precursor hasan inherent viscosity (IV), measured in NMP, at 25.0° C., and 0.2 g/dL,of 2.54 dL/g.

What is claimed is:
 1. A method for preparing a polyimidebenzoxazole polymer comprising:(a) reacting an aromatic dianhydride, an aromatic diaminobenzoxazole and a bifunctional chain extender to form a polyimide precursor, one functional group of the chain extender being reactive with the amine of the aromatic diaminobenzoxazole or the anhydride of the aromatic dianhydride and the other functional group being an unsaturated group or a strained aromatic ring that do not form amic acid linkages, said reacting being performed in the presence of an aprotic polar solvent and at most about 5 mole percent water based on the total moles of dianhydride, aromatic diaminobenzoxazole and chain extender; (b) curing the polyimide precursor to a conversion temperature of about 160° C. to about 280° C. for a time sufficient to convert at least a portion of the precursor to polyimidebenzoxazole and (c) further heat treating at(i) one temperature of at least about 350° C. to at most about 600° C. or (ii) two or more successively higher temperatures that are higher than the conversion temperature to at most about 600° C. and at least one temperature is at least about 260° C.,to form a polyimidebenzoxazole polymer having a tensile strength of at least about 35 Ksi.
 2. The method of claim 1 wherein the reacting step is performed essentially free of water.
 3. The method of claim 1 wherein the aromatic dianhydride is of the formula: ##STR31## and the aromatic diaminobenzoxazole is of the formula: ##STR32## where Ar, Ar¹, Ar², Ar³, and Ar⁴ are an aromatic group or pyridine group and the bifunctional chain extender is H₂ N--M--C(R)═C(R)₂, H₂ N--M--C.tbd.C(R), ##STR33## where M is a divalent organic radical; and R is hydrogen, methyl or phenyl.
 4. The method of claim 3 wherein Ar is: ##STR34## where T is --O--, --CH═CH--, --S--, --CF₂ --, --SO₂ --, --CH₂ --, ##STR35##
 5. The method of claim 3 wherein the aromatic diaminobenzoxazole is of formula (a) or (b) and Ar¹ is: ##STR36## where T¹ is --O--, --S--, --SO₂ --, --S(═O)--, --CH₂ --, ##STR37## and each Ar² are the same or different and are: ##STR38##
 6. The polyimide precursor of method of claim 3 wherein the aromatic diaminobenzoxazole is of formula (c), Ar² is: ##STR39## and T² is --O--, --SO₂ --, --S--, --S(═O)--, --CH₂ --, ##STR40## and Ar³ is: ##STR41##
 7. The method of claim 3 wherein the aromatic diaminobenzoxazole is of formula (d), each Ar³ is: ##STR42## T² is --O--, --SO₂ --, --S--, --S(═O)--, --CH₂ --, ##STR43## and Ar⁴ is: ##STR44## where T⁴ is: --S--, --O--, --SO₂ --, CH₂ --, ##STR45##
 8. The method of claim 1 wherein the diaminobenzoxazole is 2,6-(4,4'-diaminodiphenyl)benzo 1,2-d:5,4-d'!bisoxazole; 5-amino-2-(p-aminophenyl)benzoxazole; 5-amino-2-(m-aminophenyl) benzoxazole; 4,4'-diphenylether-2,2,'-bis(5-aminobenzoxazole); 2,2'-p-phenylene bis(5-aminobenzoxazole); or 2,2-bis(4-phenyl)hexafluoropropane-2,2'-bis(5-aminobenzoxazole).
 9. The method of claim 5 wherein the aromatic dianhydride is pyromellitic dianhydride; 4,4'-oxydiphthalic anhydride; 2,2-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride; 3,3',4,4'-biphenyl tetracarboxylic dianhydride; 3,3',4,4'-diphenyl sulfone tetracarboxylic dianhydride; or 3,3',4,4'-benzophenonetetracarboxylic dianhydride.
 10. The method of claim 1 wherein the aromatic dianhydride is pyromellitic dianhydride and the diaminobenzoxazole is 5-amino-2-(p-aminophenyl)benzoxazole.
 11. The method of claim 1 wherein the bifunctional chain extender is maleic anhydride, nadic anhydride, vinyl phthalic anhydride, 1,2-dimethylmaleic anhydride, 3-(3-phenylethynlphenoxy) aniline, phenylethynlaniline, ethynylaniline, propargyl amine, aminobenzocyclobutene or other aromatic cyclobutene functionalized with an amine or anhydride reactive group.
 12. The method of claim 10 wherein the bifunctional chain extender is maleic anhydride, 4-aminobenzocyclobutene, nadic anhydride, propargyl amine, and phenylethynlaniline.
 13. The method of claim 1 wherein the polar aprotic solvent is N,N'-dimethyl-acetamide, N-methylpyrrolidinone, 1,3-dimethyl-5-imidazolidinone, N,N-dimethylformamide, 1,1,3,3-tetramethylurea, or N-cyclohexylpyrrolidinone.
 14. The method of claim 13 wherein the reacting step is performed in the presence of a non-solvent for the polyimide precursor, the non-solvent being miscible with the polar aprotic solvent and being present in an amount of up to 0.65 volume fraction by volume of the non-solvent and polar aprotic liquid.
 15. The method of claim 14 wherein the polar aprotic solvent is N-methyl-pyrrolidinone.
 16. The method of claim 1 wherein the further heat-treating step comprises an annealing temperature from about 250° C. to about 400° C. for from about 0.1 to about 120 minutes and then to a heat-treating temperature, higher than the annealing temperature, from about 260° C. to about 600° C. for from about 0.1 to about 120 minutes.
 17. The method of claim 1 wherein the one temperature of the further heat-treating step is maintained for about 0.1 to 120 minutes.
 18. The method of claim 1 wherein the polyimidebenzoxazole is a film formed by shaping the liquid film comprised of the polar aprotic solvent and polyimide precursor of step (a).
 19. The method of claim 18 wherein the liquid film is contacted with a ring-closure agent prior to or while at a temperature sufficient to imidize the polyimide precursor.
 20. A polyimidebenzoxazole film comprising, in polymerized form, pyromellitic dianhydride, 5-amino-2-(p-aminophenyl)benzoxazole and benzocyclobutene.
 21. The polyimidebenzoxazole film of claim 20 wherein the film has a tensile strength of at least about 35 Ksi.
 22. A polyimidebenzoazole film comprising, in polymerized form, 4,4'-oxydiphthalic anhydride, 2,6-(4,4'-diaminophenyl)benzo 1,2-d:5,4-d'!bisoxazole and maleic anhydride.
 23. The film of claim 22 wherein the film has a tensile strength of at least about 35 Ksi. 